p(x) = x4 - x3 - 5x2 + 3x + 6
⇔p(-1) = 1 + 1 - 5 - 3 + 6 = 0 ⇒x + 1 is a factor
p(2) = 16 - 8 - 20 + 6 + 6 = 0 ⇒ x - 2 is a factor
⇒(x4 - x3 - 5x2 + 3x + 6)/(x2 - x - 2) = x2 - 3
⇒x4 - x3 - 5x2 + 3x + 6 = (x2 - x - 2)(x2 - 3) = 0
⇒(x - 2)(x + 1)(x2 - 3) = 0
⇒x = -1, 2, root of 3, - root of 3